Surface modified calcium carbonate composition and uses therefor

ABSTRACT

An improved form of calcium carbonate which is acid resistant to enable its use as a filler material in the making of neutral to weakly acid paper, and a process for producing this acid-resistant calcium carbonate is provided. This acid-resistant calcium carbonate comprises calcium carbonate in admixture with at least about 0.5 to about 10 percent, based on the dry weight of the calcium carbonate, of a cationic salt, together with at least about 0.5 to about 10 percent, based on the dry weight of the calcium carbonate, of an anionic salt.

This application is a divisional of application Ser. No. 08/518,757,filed Aug. 24, 1995, now U.S. Pat. No. 5,531,821.

BACKGROUND OF THE INVENTION

This invention relates generally to calcium carbonate for use inpapermaking, and related industries, and more particularly to a calciumcarbonate having acid resistant properties.

Titanium dioxide and calcined clay have traditionally been utilized asfiller materials in the preparation of neutral to weakly acidic paper inorder to improve the optical properties, especially the brightness, ofthe resultant product. These materials, however, especially titaniumdioxide, have the disadvantage of being very expensive, resulting inhigher manufacturing costs and an uncompetitively priced paper product.

Calcium carbonate, particularly precipitated calcium carbonate, has beenused as a filler material in the making of alkaline paper. Such usageresults in a paper with enhanced optical properties, without the expenseincurred in using titanium oxide fillers, resulting in a much lessexpensive product. Calcium carbonate, however, cannot generally be usedas a filler in acidic paper because it decomposes in an acidicenvironment. Consequently, there has long been a need to develop acalcium carbonate composition which is acid stabilized and resistant todecomposition at low pH, so that it can be utilized as a filler materialin the manufacture of acidic paper, such as groundwood paper, where theuse of an alkaline filler would have a negative impact on the finalpaper properties.

U.S. Pat. No. 5,043,017 discloses and claims an acid-stable calciumcarbonate resistant to degradation in a mildly acidic environment whichcomprises a mixture of a calcium-chelating agent or a conjugate base,and a weak acid such that calcium carbonate is coated by and is inequilibrium with the calcium-chelating agent or conjugate base and theweak acid. Preferred calcium carbonate compositions contain sodiumhexametaphosphate and phosphoric acid.

U.S. Pat. No. 5,164,006 discloses and claims an acid resistant calciumcarbonate which is prepared by preparing an aqueous slurry of calciumcarbonate, heating the slurry to about 75°-80° C., slowing adding sodiumsilicate solution in an about of about 5-10% by weight, adding gaseouscarbon dioxide, cooling the slurry and adding zinc chloride to theslurry to bring the pH to a range of about 7.5 to 8.0.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide an acid resistantcalcium carbonate composition especially suitable for use in papermakingapplications.

It is a further object of the present invention to provide a process forthe preparation of the aforesaid calcium carbonate compositions.

A still further object of the present invention is to provide a paperhaving enhanced optical qualities prepared using the calcium carbonatecompositions of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to an improved form of calcium carbonatewhich is acid resistant to enable its use as a filler material in themaking of neutral to weakly acid paper, and a process for producing thisacid resistant calcium carbonate. More particularly, this inventionconcerns an acid resistant calcium carbonate comprising a mixture ofcalcium carbonate with at least about 0.5 to about 10 percent, based onthe dry weight of the calcium carbonate, of a cationic salt, togetherwith at least about 0.5 to about 10 percent, based on the dry weight ofthe calcium carbonate, of an anionic salt. It has surprisingly beenfound that the inclusion of the both a cationic salt and an artionicsalt with the calcium carbonate confers a higher degree of acidresistance for calcium carbonate in the presence of fiber slurry, and alonger term of low pH stability than untreated calcium carbonatecompositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the 24 hour aging of ground calcium carbonatecompositions of the present invention using various concentrations ofsodium hexametaphosphate and aluminum sulfate.

FIG. 2 is a graph showing the 24 hour aging of ground calcium carbonateusing various concentrations of sodium hexametaphosphate and aluminumchloride.

FIG. 3 is a graph showing the aging of ground calcium carbonate usingvarious concentrations of sodium hexametaphosphate and aluminumchloride.

FIG. 4 is a graph showing the aging of scalenohedral precipitatedcalcium carbonate using sodium polyacrylate and various concentrationsof polyaluminum chloride.

DETAILED DESCRIPTION OF THE INVENTION

The improved form of calcium carbonate prepared by the instant inventionis acid resistant to enable its use as a filler material in the makingof neutral to weakly acid paper. While not wishing to be bound by anyparticular theory as to the operability of the present invention, it isbelieved that the acid resistance conferred upon the calcium carbonatecompositions of the present invention is a result of the inactivation ofthe surface of the calcium carbonate by the addition of the weak baseand the weak acid.

In the practice of the present invention, the calcium carbonatecompositions are rendered acid resistant by the inclusion of at leastabout 0.5 to about 10 percent, based on the dry weight of the calciumcarbonate, of a cationic salt together with at least about 0.5 to about10 percent, based on the dry weight of the calcium carbonate, of ananionic salt.

The cationic salt utilized in the compositions of the present inventionis typically an aluminum salt. Included are salts such as aluminumammonium sulfate, aluminum potassium sulfate, chromium potassiumsulfate, aluminum chloride, polyaluminum chloride and chromium ammoniumsulfate. Preferably, aluminum sulfate, aluminum chloride, andpolyaluminum chloride are utilized as the cationic salt.

The amount of the cationic salt utilized is at least 0.5 percent toabout 10 percent, based on the dry weight of the calcium carbonate, andis preferably about 2 to about 7 percent, based on the dry weight ofcalcium carbonate.

The anionic salt utilized in the compositions of the present inventionare salts of weak bases such as salts of hexametaphosphoric acid, saltsof an organic polymeric acid, salts of acetic acid, salts of oxalicacid, salts of phosphoric acid, salts of metaphosphoric acid, salts ofcitric acid, salts of boric acid, salts of sulfurous acid, salts ofethylenediamine tetraacetic acid (EDTA), salts of polymaleic acid, saltsof polycarboxylic acid, and mixtures thereof. Preferred anionic saltsare sodium hexametaphosphate, sodium polyacrylate and sodiumpolymaleate.

The amount of the anionic salt utilized is at least 0.5 percent to about10 percent, based on the dry weight of the calcium carbonate, and ispreferably about 1 to about 5 percent, based on the dry weight ofcalcium carbonate.

The calcium carbonate utilized is preferably finely divided and it canbe either a precipitated calcium carbonate or a natural groundlimestone.

The process for producing this acid resistant calcium carbonate is muchsimpler than the process described by the aforementioned U.S. Pat. No.5,164,006, and involves first forming a mixture of calcium carbonatewith at least about 0.5 to about 10 percent, based on the dry weight ofthe calcium carbonate, of the anionic salt to be utilized. Then, atleast about 0.5 to about 10 percent, based on the dry weight of thecalcium carbonate, of the desired cationic salt is added to thisresultant mixture. Finally, the resultant mixture is blended for asufficiently long period of time to ensure uniform mixing of theingredients.

The calcium carbonate can be utilized in the above-described processeither as a dry powder or an aqueous slurry with up to about 60 percentby weight solids content.

The cationic salt can be utilized in the instant process either as a drysolid or and an aqueous solution. When the calcium carbonate is used indry powder form, it is preferable to utilize an aqueous solution of theweak base in order to facilitate homogeneous mixing. Where a slurry ofthe calcium carbonate is utilized, the solid form of the cationic saltreadily dissolves therein so that an aqueous solution is unnecessary.

The anionic salt can be utilized in the process of preparation in eithera pure concentrated form or as an aqueous solution.

The composition of the present invention can be utilized to improve theoptical properties of neutral to weakly acidic paper by its addition tothe paper during standard manufacturing processes. Typically, thecalcium carbonate composition of the present invention is added to afirst paper furnish containing components necessary for making acidicpaper to thereby form a second paper furnish. The invention will befurther illustrated by the following Examples, which are to beconsidered illustrative of the invention, and not limited to the preciseembodiments shown.

EXAMPLE 1

Ground Calcium Carbonate Composition

Acid resistant ground calcium carbonate slurry can be obtained by theaddition of a anionic salt such as sodium hexametaphosphate, and acationic salt such as aluminum sulfate. Initially, 1-5% sodiumhexametaphosphate, based on the dry weight of calcium carbonate, wasadded into a slurry of ground calcium carbonate having an initial pH of8.01. After mixing, 1-7% aluminum sulfate, based on the dry weight ofthe calcium carbonate, was added. A plot of the pH was measured for eachsample after 24 hours ageing, and the results are shown in the graphshown in FIG. 1. From FIG. 1, the line connecting the oval pointsindicates that the calcium carbonate slurry prepared from 1% sodiumhexametaphosphate, based on the dry weight of calcium carbonate, and 3%alum, based on the dry weight of calcium carbonate, resulted in aproduct that had a pH of 6.24 after 24 hours ageing.

EXAMPLE 2

Ground Calcium Carbonate Composition

Acid stabilized ground calcium carbonate slurry can alio be produced bycharge neutralization coating with aluminum chloride and sodiumhexametaphosphate. The pH of ground calcium carbonate slurry is 8.01. Atthe beginning, 1%, 3% and 5% sodium hexametaphosphate, based on the dryweight of calcium carbonate, was added, followed by the addition of 5%aluminum chloride, based on the dry weight of calcium carbonate. A plotof pH was measured for each sample after 24 hours ageing as shown inFIG. 2 and FIG. 3. From the results, 2% sodium hexametaphosphate, basedon the dry weight of calcium carbonate, was added. After blending, 5%aluminum was added, based on the dry weight of calcium carbonate.Initially, the pH of ground calcium carbonate slurry was 5.70 andresulted in a stable pH which was 5.85 after 24 hours ageing.

EXAMPLE 3

Ground Calcium Carbonate Composition

The pH of ground calcium carbonate slurry is 8.01. First, three percentsodium polyacrylate, based on the dry weight of calcium carbonate, wasadded, followed by the addition of 5% polyaluminum chloride, based onthe dry weight of calcium carbonate. Initially, the pH of ground calciumcarbonate slurry was 6.12 and resulted in a stable pH which was 6.60after 116 hours ageing.

EXAMPLE 4

Scalenohedral Precipitated Calcium Carbonate Composition

Acid stabilized scalenohedral calcium carbonate slurry can also beproduced by charge neutralization coating with aluminum chloride andsodium polyacrylate. The pH of scalenohedral calcium carbonate slurry is9.05. Initially, 1% sodium polyacrylate, based on the dry weight ofcalcium carbonate, was added, followed by the addition of 2%, 4% and 6%polyaluminum chloride, based on the dry weight of calcium carbonate. Aplot of pH was measured for each sample after 24 hours ageing as shownin FIG. 4. From the results, the pH of scalenohedral precipitatedcalcium carbonated slurry was 6.02 with the treatment of 1% sodiumpolyacrylate/6% polyaluminum chloride, based on the dry weight ofcalcium carbonate, and the final pH of the slurry was found to be 6.44after 68 hours ageing.

EXAMPLE 5

Rhombic Precipitated Calcium Carbonate Composition

Acid resistant rhombic precipitated calcium carbonate slurry can beobtained by the addition of a anionic salt such as sodium polyacrylate,and a cationic salt such as aluminum sulfate. Initially, 2% sodiumpolyacylate, based on the dry weight of calcium carbonate, was addedinto a slurry of rhombic precipitated calcium carbonate having aninitial pH of 8.09. After mixing, 4% aluminum sulfate, based on the dryweight of the calcium carbonate, was added. Initially, the pH of thiscomposition was 6.41, and after 24 hours ageing, the pH was found to be6.59.

What is claimed is:
 1. A process for the preparation of an acidresistant calcium carbonate which comprises:a) forming a mixture ofcalcium carbonate with at least about 0.5 percent to about 10 percent,based on the dry weight of the calcium carbonate, of a cationic salt; b)adding at least about 0.5 percent to about 10 percent, based on the dryweight of the calcium carbonate, of anionic salt to the mixture to forma final mixture; and c) blending the final mixture to ensure uniformmixing.
 2. The process according to claim 1 wherein the anionic salt issodium hexametaphosphate.
 3. The process according to claim 1 whereinthe cationic salt is aluminum sulfate.
 4. The process according to claim1 wherein the cationic salt is present in an mount of about 2 to about 7percent, based on the dry weight of the calcium carbonate.
 5. Theprocess according to claim 1 wherein the anionic salt is present in anamount of about 1 to about 5 percent, based on the dry weight of thecalcium carbonate.